Nuclear plant safety . •! Surface deposition at four Swedish sites (kilobecquerel per square metre) Monitoring the fallout Radionuclide by Mikael Jensen and John-Christer Lindhe Zirconium-95 1.5 0.7 1.0 0.2 Niobium-95 1.4 0.8 1.4 0.2 Molybdenum-99 0.8 1.3 4.4 1.2 Ruthenium-103 1.0 2.1 7.7 2.2 50 In Sweden fallout from the Soviet reactor in Chernobyl was lodine-131 6.7 170 27 Tellurium-132 1.6 22 210 13 first detected on 28 April when personnel from the morning Caesium-134 0.2 6.5 24 6.5 shift at the Forsmark power station measured increasing Caesium-137 0.3 8.2 32 5.7 amounts of radioactivity on personnel passing the station's Barium-140 2.0 10 36 9.0 portal monitor. A check of surface contamination on the Lanthanum-140 1.8 7.8 23 6.6 2.0 ground by the station also showed increased values. Neptunium-239 5.0 2.0 The reactor site was partly evacuated, but it soon became Dose rate 0.1 1 0.8 clear that increased levels of radioactivity could be measured (microsievert per hour) all over the Swedish eastern coast. Early measurements by the Notes: The table shows preliminary results from some relatively high Swedish National Defense Research Institute (FOA) showed intensity regions. that the source of the release was not an atmospheric nuclear In Stockholm readings were taken at a lawn outside the laboratory (dry bomb but rather a nuclear reactor accident. Backward air deposition); in Hallstahammar, in an open space at the edge of a forest; in Tarnsjo, in plowed soil of a high rainfall region; and in Hudiksvall, at a wet parcel trajectories are automatically calculated by the Swedish air strip. Institute of Meteorology and Hydrology and transmitted daily to FOA. That morning they indicated that the source was to be found in the direction of Lithuania, White Russia, and the Ukraine. Suspicion first fell on the large nuclear power plant at Ignalino in Lithuania, which is much closer to Sweden than About this article... Chernobyl. In Sweden radiation measurements (in microroentgen per This report is a first attempt to present an overall view of hour at ground level) during the period 1-8 May 1986 showed recommendations by the Swedish National Institute of Radiation the highest levels near the city of Gavle (400 microroentgen Protection (SSI) and of the measurements made by many Swedish authorities and organizations after the Chernobyl per hour and higher measured as the mean value over large accident. areas). Locally, values exceeded 1000 microroentgen per hour Two resources turned out to be essential for the early in smaller areas. This area experienced rainfall on Monday mapping of the fallout and the detailed analysis of its composition: 28 April and Tuesday morning 29 April. the network of sampling stations for ground-level air run by the Swedish National Defense Research Institute (FOA), and the high analysis capability maintained by its Nuclear Detection Countermeasures taken Laboratory. This laboratory also carried out high altitude sampling with help from the Air Force. Its In Situ Gamma Group also was The Swedish National Institute of Radiation Protection transported around the country by Navy Helicopters for reliable (SSI) initially issued recommendations against drinking rain- and detailed deposition measurements in the early phase. water collected from open surfaces and a few other minor Other especially useful resources were the SSI's monitoring restrictions. However, it soon became clear that the sole stations throughout Sweden to measure deposition on the restriction deemed necessary at the time was keeping cows ground, and the big scintillation detector system flown across the country by the Swedish Geological Company (SGAB). Together, indoors in regions with grass contents of caesium higher than these produced a very good overall picture of the radiation 1 kilobecquerel per square metre, which corresponds roughly pattern. to a total ground concentration of 3 kilobecquerel per square Furthermore, measurements were carried out by several metre. Measurements on milk from the test farms where cows external bodies, including laboratories at Swedish nuclear power stations and universities. grazed in areas with high concentrations of caesium showed Time has not permitted SSI to closely examine all data, nor that this was a very conservative estimate giving a more than can we guarantee that the organizations responsible for different ample margin to meet the limit of 300 becquerel per litre of measurements reported to SSI have had proper time to evaluate and doublecheck their data. Detailed results and descriptions of measurement conditions will have to follow later. Messrs Jensen and Lindhe are with the Swedish National Institute of Radiation Protection (SSI) in Stockholm. IAEA BULLETIN, AUTUMN 1986 caesium in milk and foodstuffs. It was also found that the con- tent of caesium fell as new grass grew up. Gradually, restric- tions were removed and on 25 June cows could again graze freely all over Sweden. Many activities remain to be followed up. Also, meat from wildlife and reindeer (predominantly slaughtered in the autumn) may exceed the limit. It was found that routine occupational health procedures often met the radiation protection requirements. One example is the normal protective measures taken against dust during changes of large industrial air filters. Radiation monitoring: The air FOA continuously measures radionuclides in air at ground stations using forced ventilation through glass-fiber filters, and by aircraft equipped with air filters.* Following Chernobyl, ground-level filters were changed much more frequently than normal, in periods of every 3 hours. Each day FOA and the Air Force analysed the radionuclide concentration at some altitude between 100 and 800 metres on a route chosen from expected cloud trajectories. Iodine-131 and other radionuclides were present in the air at ground level. The iodine collected on the filter corresponds to a maximum concentration of about 10 becquerel per cubic metre for the Stockholm area.** Parts of the filters have been autoradiographed to identify hot particles. Gamma spectroscopy studies of such identified Sweden's radiation monitoring stations particles show an expected enhancement of refractive elements, such as cerium, ruthenium, zirconium, and others. At the end of the 1950s, Sweden established a system of some 25 stations to continuously monitor gamma radiation from Single hot particles identified on the ground by simply using the ground due to fallout from atmospheric testing of nuclear a gamma sensing instrument have turned out to emit radiation weapons. As the weapon fallout has diminished, these stations from almost purely ruthenium and molybdenum radionuclides. have served to follow variations in natural background radiation. Now in 1986, they have proved to be very useful as one means of following levels of gamma radiation over the country after a severe nuclear accident. The stations are under the auspices of Aircraft surveys the Swedish National Institute of Radiation Protection (SSI). These stations use an ionization chamber placed 2.5 metres Measurements also were made by the Swedish Geological above the ground. They register gamma radiation from the Company in aircraft flown 150 metres above the ground. ground, as well as the constant contribution from cosmic radia- These measurements have been used to give a general first tion. The detector cannot distinguish between gamma radiation picture that was later followed by grass sampling and measure- from the ground and from the air. In the case of low air activity after the passage of the plume from Chernobyl, the detector ments by mobile patrols (gamma spectrometry). (Such surveys reading reflected mainly the ground deposition. also are considered valuable in emergency planning in the Information from these stations has been a valuable indicator event of Swedish reactor accidents. They are expected to be of the general trend all over the country. Information from at least launched in the intermediate period within a few days after an some stations was available each morning, which enabled accident). The aircraft carry equipment for gamma spectrome- authorities to report the trend during the previous night. try that can be used to describe the distribution of individual gamma-emitting nuclides. 28 April. Later that night, this cloud was washed out by rain and produced high deposition areas south of Gavle.* Site measurements In situ measurements (employing a high-resolution ger- Grass measurements manium detector looking down from one metre above ground) were carried out by FOA continuously at the Laboratory in The significance of measuring grass samples stemmed from Stockholm and with their mobile system at a number of places the decision recommending that cattle be kept in stables until between Malmo and Rudiksvall (See accompanying table). readings showed acceptable levels. This level was set at The mobile spectroscopy system also made measurements 10 kilobecquerel per square metre of iodine-131 (correspond- in the plume above the Baltic Sea during the evening of ing to an expected milk concentration of about 2 kilobecquerel per litre). Grass samples were taken and evaluated by laborato- ries of Swedish nuclear power plants, the universities of Lund * For a description, see "The Swedish Air Monitoring Network for Particu- and Uppsala, and Studsvik Energiteknik AB. late Radioactivity", in IEEE Transactions on Nuclear Science, Vol. NS-29, No. 1, (February 1982) Extract. * The cloud, and its resulting wet deposition, had a different nuclide composi- ** To calculate doses from such measurements, a proper factor must be applied tion compared to the dry deposition. Most important was the fivefold increase to account for iodine passing through the filter. A comparison with other in the tellurium-132/iodine-l31 ratio in the wet deposition. As the half-life of measurements indicates that a value for this factor is perhaps 3, not higher than tellurium is rather short (3.25 days), this means that fallout fields of high density 5.